Explanation
'Cannot do without...', 'unavoidable', 'have to'.
Formation
Verb-ない form (remove ない) + ずにはすまない
Detailed explanation
The grammar point ずにはすまない is used when something must be done due to social/moral obligation, necessity, or rules. It implies that there is no way around taking that action.
Examples
Kono mondai o kaiketsu shinai to, shakai zentai ga eikyō o ukeru node, kaiketsu sezu ni wa sumanai.
Since this issue will affect the entire society if not resolved, we cannot do without solving it.
Kore wa kodomo-tachi no mirai o omou to, kyōiku kaikaku o okonawazu ni wa sumanai koto da.
For the sake of the children’s future, we cannot avoid carrying out educational reforms.
Kono ken ni tsuite wa, kare ni ayamarazu ni wa sumanai.
Regarding this matter, I can’t do without apologizing to him.
Kekkonshiki ni shusseki suru nara, purezento o watasazu ni wa sumanai.
If you attend a wedding, you have to bring a gift.
Practice
Grammar content sourced from hanabira.org.